1. Field of the Invention
The present invention relates to a zoom lens barrel.
2. Description of the Related Art
A zoom lens barrel having a diaphragm, which is movable in the optical axis direction, the aperture diameter of which is fixed and which limits the amount of light passing therethrough depending on the position of the diaphragm in an optical axis direction, is known in the art. Japanese Unexamined Patent Publication 2009-192815 shows a support structure for such a diaphragm. In this support structure, the diaphragm is positioned between a first lens group frame and a second lens group frame, a spring member and another spring member are installed between the first lens group frame and the diaphragm and between the diaphragm and the second lens group frame, respectively, and the diaphragm is supported in a floating state (suspended state) by balancing the spring forces of these two spring members during zooming.
However, the spring forces of identical spring members usually vary by some degree (e.g., ±15%) due to manufacturing error, and accordingly, in Japanese Unexamined Patent Publication 2009-192815, the spring forces of the two spring members cannot be equally balanced out at all times during zooming, so that the position of the diaphragm in the optical axis direction cannot be determined with precision. Consequently, the amount of light passing through the diaphragm cannot be controlled with precision, which makes it impossible to achieve a desired diaphragm effect (stop-down effect).